Many internal combustion engines use various methods and systems for regulating engine speed so as to avoid engine overspeed conditions. Engine overspeed occurs when the engine is operating at high speed, such as wide-open-throttle, and some workload is suddenly removed from the engine, such as when a blade of an engine-powered chainsaw finally breaks through a log it is cutting. Among the options for regulating engine speed, some engine designs incorporate fuel flooding, ignition timing retard, or ignition suppression.
With any of these options, a spark-ignition engine cycle includes a compression stroke wherein a piston compresses an air-fuel mixture within an engine combustion chamber, which is defined by an engine cylinder and a top surface of the piston. The cycle also includes an ignition event wherein a spark plug ignites the compressed air-fuel mixture, typically when the piston is rising at a predetermined point with respect to a “top dead center” (TDC) position within the cylinder. The ignition event initiates a combustion event in which chemical energy of the air-fuel mixture is converted into thermal energy. Subsequently, the thermal energy is converted into mechanical work during a power stroke of the cycle, wherein the combustion event rapidly expands the gas volume and increases the pressure within the combustion chamber, thereby forcing the piston down away from TDC. Consequently, the linear displacement of the piston during the power stroke is converted into rotation of a crankshaft via a pivotable connecting rod.
Timing of the ignition event is an important aspect in the performance of internal combustion engines and relates to how early or late a spark plug fires relative to the location of the piston within the cylinder in reference to TDC. Because there is a slight delay between ignition and peak combustion, if ignition occurs when the piston is at TDC, the piston will have already moved well down into its power stroke before combustion gases have achieved their highest useful pressure. Therefore, to make the most efficient use of the chemical energy of the fuel, ignition should occur before the piston reaches TDC during its compression stroke. But the speed of the piston increases with overall engine speed, even though the combustion time is about constant. Therefore, the faster the engine speed, the earlier ignition needs to occur relative to the TDC position of the piston to time maximum combustion pressure levels for optimum engine performance.
For instance, when the engine is operating at relatively high speeds it is desirable to initiate combustion well before the piston reaches TDC, such that peak combustion pressure occurs immediately after the piston reaches TDC for maximum performance and efficiency. This occurrence is commonly referred to as ignition timing advance. Conversely, if the engine is being operated at relatively low speeds, it is desirable to initiate combustion when the piston is closer to TDC such as slightly before or slightly after TDC. Moreover, ignition timing is “advanced” or “advancing” whenever timing is being adjusted relatively away from TDC toward a piston compression position that is before top dead center (BTDC). Conversely, ignition timing is “retarded” or “retarding” whenever timing is being adjusted in a direction generally defined as progressing relatively from BTDC toward ATDC.
Engine overspeeding is a condition that can be regulated during engine cycles that exceed a predetermined high speed threshold, in accord with the several options mentioned above. According to the first option, the air-fuel mixture can be enriched so as to flood the combustion chamber with fuel and thereby partially or completely extinguish ignition. Once engine speed falls to an acceptable level, the air-fuel mixture can be normalized. Unfortunately, however, this method can be difficult to control and yields increased unburned fuel emissions that are exhausted out of the engine. According to the second option, ignition timing can be retarded closer to TDC during all overspeed engine cycles until engine speed falls to an acceptable level. But this method typically occurs over an unacceptable number of engine cycles and yields engine inefficiency and high exhaust gas temperatures, which can harm various components of the engine.
With the third option, ignition can be suppressed during overspeed engine cycles, such as by intermittent ignition or ignition cutoff. Once engine speed falls to an acceptable level, ignition can be normalized or reactivated. In the meantime, however, more and more fuel tends to accumulate in the combustion chamber and, once ignition is reactivated, combustion tends to be intensified by the accumulated fuel. Such combustion yields undesirable spikes in pressure in the combustion chamber that can be damaging to engine components and that otherwise create undesirable noise, vibration, excessive engine heating, high exhaust gas temperatures, and harshness in engine operation.
In sum, current approaches at engine speed limiting and recovery are not yet fully optimized for fuel efficiency, engine integrity, and smooth engine operation.